Rheology modifiers, also referred to as thickeners or viscosifiers, are ubiquitous in surfactant containing personal care cleansing formulations. Rheological properties (e.g., viscosity and flow characteristics, foamability, spreadability, and the like), aesthetic properties (e.g., clarity, sensory effects, and the like), mildness (dermal and ocular irritation mitigation), and the ability to suspend and stabilize soluble and insoluble components within a surfactant based formulation are often modified by the addition of a thickener.
Often, thickeners are introduced into surfactant formulations in solid form and mixed under conditions effective to dissolve the thickener into the liquid surfactant composition in order to effect a viscosity enhancement. Frequently, the mixing must be conducted at elevated temperatures (hot processing) in order to promote the dissolution of the solid thickener and obtain the desired viscosity improvement. Additionally, solid thickeners (e.g., carbomer powders) are known to resist “wet-out” upon contact with the surface of an aqueous based system. Consequently, carbomers are supplied as finely divided powders and/or must be sifted to reduce particle size which aids in dissolution by increasing the relative surface area of the particle. During processing, carbomer powders can become electrostatically charged as they are transferred in and out of containers and tend to adhere to oppositely charged surfaces including airborne dust, necessitating specialized dust extraction equipment. This means that preparation of aqueous dispersions is messy and time-consuming unless special precautions and expensive equipment is employed. Formulators of compositions containing thickened surfactant constituents desire the ability to formulate their products at ambient temperatures (cold processing). Accordingly, formulators desire thickeners which can be introduced to the liquid surfactant compositions in liquid form rather than as a solid. This provides the formulator with a greater degree of precision in introducing the thickener to the liquid surfactant composition, allows the ability to formulate products at ambient temperatures (cold processing), and better facilitates automated processing without the need for special safety and handling equipment.
One important class of liquid rheology modifier commonly employed to thicken aqueous based surfactant containing formulations is the alkali-swellable or alkali-soluble emulsion (ASE) polymers. ASE polymers are linear or crosslinked copolymers that are synthesized from (meth)acrylic acid and alkyl acrylates. The crosslinked polymers immediately thicken upon neutralization with an inorganic or an organic base. As liquid emulsions. ASE polymers are easily processed and formulated into liquid surfactant containing formulations by the product formulator. Examples of ASE polymer thickened surfactant based formulations are set forth in U.S. Pat. No. 6,635,702; International Published Application No. WO 01/19946; and European Patent No. 1 690 878 B1, which disclose the use of a polymeric thickener for aqueous compositions containing surfactants.
As the industry desires new thickened surfactant based products that are formulated at a broad pH range, there is a developing need for a rheology modifier that, when used in combination with a surfactant, provides a high clarity composition when formulated in a broad pH range while maintaining a good viscosity/rheology profile, suspension (yield value), and enhanced aesthetics (clarity).